Automatic test equipment for checking printed circuit boards has long involved the use of "bed of nails" test fixtures on which the circuit board is mounted during testing. A typical test fixture includes a large number of nail-like spring-loaded test probes arranged to make electrical contact between measurement channels in the test equipment and designated test points on the circuit board under test, also referred to as the unit under test or "UUT". Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the arrangement of test probes for contacting test points on the board must be customized in a test fixture for that particular circuit board.
Board design and fabrication data is used to determine what specific board features are to be tested by the fixture. A grid test fixture is typically fabricated by drilling patterns of holes in several rigid and nonconducting plates assembling those plates with suitable fasteners and spacers to maintain said plates in a parallel, aligned position, and then mounting test pins or probes in the drilled holes. In a "determined" grid test fixture each plate has a hole pattern which is unique such that the test pin can only be inserted to provide an x, y and z translation between a unique feature on the UUT and a unique tester grid channel.
In preparation for test, the circuit board is then positioned on the fixture precisely aligned with the array of test probes. During testing, the pins in the fixture are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred between the board and the tester through the fixture so that a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board can perform the actual test.
Referring to FIGS. 1-3, in one such conventional translator pin fixture, the translator plates 25A, 26A, 28A, 30A, 31A, 32A are held together in spaced apart, essentially parallel relationship to each other by a plurality of rigid, cylindrical support post assemblies 10 mounted normally to the plates. Typically, each support post assembly 10 consists of an elongated shaft or rod 11 which extends through coaxial holes 16A, 16B, 16C and 16D drilled in the intermediate plates 26A, 28A, 30A and 31A, respectively, i.e., the plates in-between the top plate 20A and bottom plate 20B of the fixture. A plurality of stacked cylindrical spacers or washers 12A, 12B, 12C, 12D and 12E are mounted on rod 11. The diameter of rod 11 is slightly smaller than the diameters of the coaxial holes through which it extends, while the diameters of spacers 12A, 12B, 12C, 12D and 12E are larger than the diameters of the coaxial holes 16A, 16B, 16C and 16D whereby the plates are spaced apart, in essentially parallel relationship to each other. The fixture is held together by screws 22A, 22B through holes in the top and bottom plates and into threaded holes (not shown) in the top and bottom ends of rod 11.
Unfortunately, constructing the aforesaid type of conventional translator fixture is difficult, slow, and requires a significant amount of manual labor and parts inventory. These factors can significantly increase the expense and time of manufacturing this type of conventional translator fixture.
Another type of prior art translator pin fixture is shown in FIGS. 4-8. In this type of fixture 15, the translator plates 25, 26, 28, 30, 31, 32 are held together in spaced apart, essentially parallel relationship with each other by a plurality of support elements 100. Each of the support elements 100 has a planar top (proximal) end 106 and a planar bottom (distal) end 108. Ends 106, 108 include threaded holes 110, 111 for receiving screws 112 which pass through holes (collectively referred to by numeral 113) in the plates 25 and 32 so as to affix the top plate 25 and bottom plate 32 to the ends 106, 108 of each of the supports 100. As will be described more fully below, each of the support elements 100 is mounted through a respective group of coaxially aligned holes (each of which groups is collectively referred to by numeral 104) formed in the intermediate plates 26, 28, 30, 31 of the fixture 15.
In each of the groups of mounting holes 104, the holes increase in size in the direction from the top plate 25 to the bottom plate 32. In other words, in each group of coaxial holes 104, the holes (collectively referred to by numeral 126) formed in the bottom-most intermediate plate 31 are larger in diameter than the holes (collectively referred to by numeral 128) formed in intermediate plate 30, which holes 128 are larger in diameter than the holes (collectively referred to by numeral 129) formed in the next intermediate plate 28. Likewise, holes 130 formed in the top-most intermediate plate 26 are smaller than holes 128. Additionally, plate mounting holes 113 are significantly smaller in diameter than the coaxial holes 126, 128, 129, 130 formed in the intermediate translator plates 31, 30, 28, 26, respectively.
Preferably, each support element 100 is made of one-piece, molded plastic construction, and comprises a X-shaped longitudinal support rib 131 which extends from the top 106 to the bottom 108 end of the support element 100. Each support element 100 includes a plurality of disk-shaped support areas 132, 134, 136, 137 that are coaxial with the central portion 151 of the rib 131, spaced apart longitudinally along the length of the support element 100, and oriented normal to the rib 131. Support areas 132, 134, 136, 137 extend beyond the edges of the holes 126, 128, 129, 130, respectively, and engage and support the undersides of the intermediate plates 31, 30, 28, 26, adjacent to the holes 126, 128, 129, 130, respectively. A plurality of raised notches (collectively referred to by numeral 140) extend radially outward from rib 131 and are spaced apart longitudinally from the support areas 132, 134, 136, 137, by distances slightly larger than the thicknesses of the plates 31, 30, 28, and 26 so as to permit the plates to be locked snugly in-between the support areas 132, 134, 136, 137 and the notches 140.
As seen particularly in FIGS. 4 and 5, the transverse cross-sectional size of each of the support elements 100 increases in a step-wise fashion from the top end 106 to the bottom end 108 of the support element 100. Moreover, the transverse cross-sectional size of each of the support elements 100 in-between the top surface 106 and the first support area 137 is undersized compared to the holes 130 formed in the plate 26. At the first support area 136, the transverse cross-sectional size of each of the support elements 100 increases in step-wise fashion from that existing between the top end 106 and the first support area 137, so as to be oversized compared to the holes 130, but undersized compared to the holes 129. Between the first 137 and the second 136 support areas, the transverse cross-sectional size of each of the support elements 100 remains equal to that at the first support area 137. At the second 136 support area, the transverse cross-sectional size of each of the support elements 100 increases in step-wise fashion so as to be oversized compared to the holes 129, but undersized compared to the holes 128. Between the second 136 and the third 134 support areas, the transverse cross-sectional size of each of the support elements 100 remains equal to that at the second support area 136. At the third support area 134, the transverse cross-sectional size of each of the support elements 100 increases in step-wise fashion so as to be larger than the holes 128, but undersized compared to holes 126. Thereafter, the transverse cross-sectional size of each of the support elements 100 remains constant until, at the bottom-most support area 132, it increases in step-wise fashion so as to be greater than the holes 126, and remains constant thereafter to bottom end 108.
While assembly of prior art fixture 15 of FIGS. 4-8 may take somewhat less time and manual labor than the prior art fixture of FIGS. 1-3, the time and manual labor involved in construction of fixture 15 are still larger than desirable. This is due primarily to the fact that since holes 126, 128, 129, 130 each have different dimensions, a respective, differently sized hole typically must be formed in each of the plates 25, 26, 27, 28, 29, 30 and 31, in conventional fixture 15. Thus, in order to manufacture fixture 15, it is typically necessary to use multiple drilling apparatus having differently sized drill bits and/or to frequently change drill bits in order to form holes 126, 128, 129, 130, and/or inventory several different plates. Disadvantageously, this makes the expense, time, and manual labor required to manufacture fixtures larger than desirable.